Observation of possible excitonic charge density waves and metal–insulator transitions in atomically thin semimetals

In solids, the condensation of electron and hole pairs with finite momentum leads to an ordered state known as a charge density wave, in which the charge has spatial modulation. However, lattice symmetry breaking and the accompanying relocation of the ions, which changes the charge distribution, can also occur simultaneously, making it difficult to disentangle the origin of the transition. Here we demonstrate a condensed phase in low-dimensional HfTe 2 . Angle-resolved photoemission spectroscopy measurements reveal a metal–insulator transition upon lowering the temperature. The observation of an opening gap, the renormalization of the bands and the emergence of replica bands in the low-temperature regime suggests that a charge density wave formed in the ground state. Raman spectroscopy shows no sign of lattice distortion within the detection limit. A small amount of electron doping substantially raises the transition temperature due to a reduced screening effect and a more balanced carrier density for electrons and holes. Our results indicate that the formation mechanism of the charge density wave is consistent with the excitonic insulator phase in low-dimensional HfTe 2 without any structural modification.